Spin polarizabilities of the proton by measurement of Compton double-polarization observables

Abstract

The Compton double-polarization observable $\hat{I}{\pounds }_{2z}$ has been measured for the first time in the $\hat{I}”(1232)$ resonance region using a circularly polarized photon beam incident on a longitudinally polarized target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using dispersion relation and chiral perturbation theory calculations, with the former resulting in: ${\hat{I}}_{E1E1}^3=-3.18\pm 0.52$, ${\hat{I}}_{M1M1}^3=2.98\pm 0.43$, ${\hat{I}}_{E1M2}^3=-0.44\pm 0.67$ and ${\hat{I}}_{M1E2}^3=1.58\pm 0.43$, in units of $10^{-4}{\mathrm{fm}}^4$.

7 pages, 5 figures, 2 tables, accepted for publication in PRC